Migration is a tool used in seismic processing to create an accurate image of the subsurface. In migration, geophysical events, such as changes in energy, are geometrically re-located in space and/or time to the location that the event occurred in the subsurface, rather than the location that it was recorded at the surface. Thus, migration involves repositioning of return signals to show an event (layer boundary or other structure) where it is being hit by the seismic wave, rather than where it is received and recorded. Consequently, various forms of migration are standard in data processing techniques for all geophysical methods (seismic, ground penetrating radar and electromagnetic sounding, for example).
Unfortunately, the computational migration needed for large datasets acquired today is extremely demanding on modern computers, and the process is very time consuming. This is because the reliability of the migrated image and the properties of the earth model that it samples either directly or indirectly are acutely dependent on the nature of the velocity field. As a result, the process of velocity model building can be repetitive.
Further, conventional approaches to build velocity models prior to migration can be less than desirable in several ways: 1) the velocity field often bears little or no semblance to true earth velocity (this is especially problematic before drilling a well); 2) even with data gained from an actually drilled well, it can be difficult to build a three-dimensional velocity model because the velocity function from the well typically samples the velocity field only in one dimension (vertical); and 3) these velocity fields are not made to reflect the actual geology of the area in question. As a result, the image produced after migration (migrated image) often depicts subsurface features at the wrong depths, and/or the subsurface structures are not clear.
Moreover, the final velocity fields cannot be used for deriving true earth properties such as pore pressure without further conditioning of the velocity that has just gone through an elaborate and expensive process of migration. These limitations are particularly acute for subsurface imaging in complex geologic areas such as highly faulted areas, and areas with salt and basalt where there is a lack of signal and coherent events.
The process of building a rock physics guided velocity model that is consistent with local geology will yield a migrated image that will not only put the reflectors of seismic image at correct depths—even in complex geologic areas as enumerated above—but also will yield velocities that can be directly used to derive subsurface properties such as pore pressure, fracture pressure, and lithostatic or overburden pressure.
Further, sub-salt body velocity analysis can be difficult due to lack of signal and coherent events. As a result, sub-salt velocity modeling often fails to accurately account for two important effects: temperature effects and geologic time on rock physics models that can relate effective stress to velocity and the increased heat flow through salt that alters the sediment properties adjacent to salt bodies. Moreover, these difficulties related to velocity analysis for sub-salt bodies generally occur in analysis related to large-scale “hard” or “fast” geobodies, including velocity analysis for sub-basalt and sub-carbonate regions in large areal extensions and/or significant vertical depths. Rock physics models can be used to compensate for these issues by providing a template that relates porosity and effective stress to geologic time and temperature for a given lithology.
The processing and application of seismic data, and particularly to estimation of seismic velocities, that uses rock physics as a guide to build velocity models can be used to improve any technique for migration of seismic data. For seismic velocities at any depth, velocity conditioning is achieved by creating one or more rock physics based templates of expected rock velocities versus depth for a given rock type (or a mixture of various rock types) based on basic principles of rock physics and geology. These templates are then used to build appropriate one- or multi-dimensional velocity models. When these conditioned (or constrained) velocities are used as a guide function to build a final velocity model, for example, using tomographic velocity inversion and input to a chosen migration algorithm, a superior migrated image can be generated. Note that this improved migrated image not only improves the depth expression or estimation of imaged subsurface features, but furthermore, a velocity field is created that can be directly used in earth property estimation to estimate metrics such as pore pressure, fracture pressure, overburden pressure, rock and fluid types, porosity, density and other attributes as a function of either depth or two-way travel time, i.e., the round-trip travel time for a seismic signal to travel from the a source to a reflection point and back to a receiver.
This procedure is very flexible and can be used in any migration procedure, including, but not limited to, the following exemplary migration techniques: Prestack Time Migration (PSTM), Kirchhoff Prestack Depth migration (KPSDM), Prestack Depth migration (PSDM), Reverse Time Migration (RTM), Gaussian Packet Beam Migration (GPM), Wave-equation migration (WEM), and Full Wave Inversion (FWI).
Accordingly, there is a need for methods and systems that can employ faster, more efficient, and more accurate methods for building velocity models prior to migration, such as developing seismic interpretations using rock physics guided migration. Such methods and systems may complement or replace conventional methods and systems for building velocity models prior to migration.